<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<div>hello</div>
<button></button>

</body>
<script>
   var  btn=document.getElementsByTagName("button")
    btn.onmouseover=function(){
       alert("点我")
    }
</script>
</html>